Make 'omx_itch186_message_size' public

It is useful on its own right.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Fix 'struct omx_itch186_message' name

Rename 'struct omx_itch186_msg_message' to 'struct omx_itch186_message'.
The latter name is already used elsewhere.

Note that this change most likely breaks all applications depending on
Libtrading for Nordic Equity TotalView-ITCH version 1.86.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #20 from jvirtanen/features/omx-itch186-message-size

Make 'omx_itch186_message_size' public
Merge pull request #21 from jvirtanen/fixes/omx-itch186-message

Fix 'struct omx_itch186_message' name